Water-soluble adhesives are well known as in adhesive tapes comprising a support having applied thereon glue, dextrin, polyvinyl alcohol, etc. These water-soluble adhesives have no self-adhesiveness in a dry state due to lack of rubbery elasticity and viscosity and are made adhesive for the first time on application of water. Therefore, there must be a complicated wetting apparatus for an industrial use of these adhesives. In general use, for example, in stamps or envelopes, too, wetting the adhesive on every use is troublesome.
Developments have also been made on water-soluble or water-dispersible adhesives which always exhibit pressure-sensitive adhesion. For example, U.S. Pat. No. 2,838,421, JP-B-50-84578, JP-B-52-22768, JP-B-49-23813, and JP-B-2-57110 (the term "JP-B" as used herein means an "examined published Japanese patent application") disclose pressure-sensitive adhesives comprising, as a base polymer, a water-soluble polymer, such as a carboxyl-containing polymer, e.g., polyacrylic acid, or an alkali metal salt, an amine salt or an alkanolamine salt thereof, having incorporated therein a water-soluble or water-dispersible plasticizer, such as a polyether polyol, a polyhydric alcohol, or an alkanolamine salt of oxidized rosin. In all these adhesives, water-solubility or water-dispersibility is obtained by using a base polymer prepared from a monomer containing a strongly polar functional group (carboxyl group) in the molecule thereof. Since such a water-soluble or water-dispersible polymer has a high glass transition temperature (Tg) and therefore does not exhibit sufficient pressure-sensitive adhesion at room temperature when used alone, a large quantity of the above-described water-soluble or water-dispersible plasticizer, e.g., polyether polyol or a polyhydric alcohol, is added thereto to obtain desired pressure-sensitive adhesion.
On the other hand, JP-A-2-120381 (the term JP-A" as used herein means an "unexamined published Japanese patent application") suggests to use, as a base polymer, a carboxyl-containing polymer having a low Tg which is prepared from an acrylic acid-caprolactone adduct to provide a water-soluble or water-dispersible pressure-sensitive adhesive which exhibits sufficient pressure-sensitive adhesion with a reduced amount of a water-soluble or water-dispersible plasticizer.
The above-described known water-soluble or water-dispersible pressure-sensitive adhesives have been developed chiefly for use in bonding of paper or broken pieces of paper in the field of paper manufacturing. This is because adhesion workability can be improved by pressure adhesion and also because such an adhesive can easily be dissolved or re-dispersed in water when broke is reduced to pulp.
However, the conventional water-soluble or water-dispersible pressure-sensitive adhesives generally have poor water-solubility or water-dispersibility in an acidic aqueous solution of pH of around 3 since the base polymer used contains a carboxyl group in the molecule thereof. Therefore, they are of little utility for the above-described purpose in the paper manufacturing industry.
Further, where the adhesives contain a large amount of a plasticizer, the plasticizer migrates to paper to cause stains (bleed-through) during long-term preservation or under a high temperature and high humidity condition. Stains of paper due to bleeding of the plasticizer causes printing defects, adhesion defects, or appearance defects.